Excited state atomic line filters

ABSTRACT

An excited state atomic line filter. The present invention solves the problem of lack of ground state resonant lines in at wavelengths substantially longer than those of visible light. Atomic line filters of the Faraday or Voigt crossed polarizer type are provided in which alkali metal atomic vapor in a vapor cell is excited with a pump beam to an intermediate excited state where a resonant absorption line, at a desired wavelength, is available. A magnetic field is applied to the cell producing a polarization rotation for polarized light at wavelengths near the resonant absorption lines. Thus all light is blocked by the cross polarizers except light near one of the spaced apart resonant lines. However, the polarization of light at certain wavelengths near the resonant is rotated in the cell and therefore passes through the output polarizer.

The present invention related to optical filters and especially toatomic line filters (ALF).

BACKGROUND OF THE INVENTION

Atomic line filters are a class of optical filters which have acceptancebandwidths on the order of 0.001 nm. In one prior art type of ALF,broadband light containing narrowband signal light is passed through afirst color glass filter which cuts off light at wavelengths below athreshold value. The signal and remaining noise light enter an atomicvapor that only absorbs the signal light within the atom's 0.001 nmacceptance bandwidth thereby exciting those absorbing atoms to anintermediate energy level. A pump beam further excites those atoms to asecond, higher energy level that then decays through various processesincluding fluorescence, to the ground state of the atom. The emittedfluorescence occurs at wavelengths below the threshold value of thefirst color glass filter. A second color glass filter then cuts off anywavelengths above the threshold which effectively permits passage ofonly the emitted narrowband fluorescence. In effect, the incoming signalhas been internally shifted in wavelength by the atomic vapor, whichthen allows the use of two overlapping color glass filters to block anybackground radiation.

Another type of prior art atomic line filter takes advantage of eitherthe Faraday effect or the Voigt effect where an atomic vapor in amagnetic field produces polarization rotation in order to pass a narrowspectral band of light through two crossed polarizers. These filters areknown respectively as Faraday filters or Voigt filters. An important usefor these filters is to block background light so that a beacon laserbeam can be detected by a wide field-of-view detector.

Operational principles of Faraday filters can be understood by referenceto FIGS. 7A-C. Crossed polarizers 90 and 91 serve to block outbackground light with a rejection ratio better than 10⁻⁵. Because thesepolarizers only work over a limited wavelength region in the infrared, abroad band interference filter may be used in conjunction with theFaraday filter. Between the polarizers, an atomic vapor (which in manyof these filters is cesium or rubidium) in a magnetic field axiallyaligned with the path of the beam, rotates the polarization of the lasersignal by 90°, while leaving background light at other wavelengthsunrotated, and thus blocked by the polarizers.

In the case of the Faraday filter the magnetic field is applied in thedirection of the signal beam, and in the case of the Voigt filter themagnetic field is applied perpendicular to the signal beam direction andat 45 degrees to the direction of each of the two cross polarizers.

Prior art atomic line filters patents issued to co-workers of applicantincludes Pat. Nos. 4,983,844; 5,267,010; 5,502,558; 5,731,585 and6,151,340 each of which are incorporated herein by reference. The '844patent discloses a fast atomic line filter which utilizes a pump laserand a high voltage potential to produce ion pairs from atoms excited byphotons with wavelengths corresponding to a resonant frequency. Theother patents describe applications of Faraday and/or Voigt filters.

One problem with atomic line filters such as those referred to above isthat their operation depends on the existence of a good sharp resonantabsorption line near the spectral range to be filtered. Many of thesesharp resonant absorption lines are characteristic of atomic vapors andthe filters described in the above referenced patents utilize alkalimetals such as cesium and rubidium to produce these metal vapors. Thesemetals are preferred because their vapors may be produced at relativelylow temperatures. However, good absorption lines from these alkali metalvapors are generally in the visible and the near visible spectral regionsuch as 780 nm and 852 nm.

Many optical symptoms operate at wavelengths substantially longer thanthe visible and near visible. A good example is light with wavelengthsin the range of 1.5 micron. For example, fiber optic communication istypically at wavelengths in the range of about 1.2 micron to about 1.65microns (see FIG. 5). Typical long haul filter optics operate within a Cor L band. (C band is 1520 nm to 1570 nm, and L band is 1570 nm to 1620nm). Shorter range fiber optics requiring higher quality fiber opticsmay operate within an s band (1.31-1.48 microns). Also, there is a needfor filters at even longer wavelength such as about 1.5 to 5 microns forlaser tracking and for free space laser communications. Applicants havesearched for good resonance lines in the alkali metals at thesewavelengths without success.

SUMMARY OF THE INVENTION

The present invention solves the problem of lack of ground stateresonant lines in at wavelengths substantially longer than those ofvisible light. Atomic line filters of the Faraday or Voigt crossedpolarizer type are provided in which alkali metal atomic vapor in avapor cell is excited with a pump beam to an intermediate excited statewhere a resonant absorption line, at a desired wavelength, is available.A magnetic field is applied to the cell producing a polarizationrotation for polarized light at wavelengths near the resonant absorptionlines. Thus all light is blocked by the cross polarizers except lightnear one of the spaced apart resonant lines. However, the polarizationof light at certain wavelengths near the resonant is rotated in the celland therefore passes through the output polarizer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A Shows a laboratory demonstration of the present invention with acesium cell.

FIG. 1B Shows a laboratory demonstration of the present invention with arubidium cell.

FIG. 2 Is a transmission curve at approximately 1469.49 nm with thecesium cell.

FIG. 3 Is a drawing of a preferred embodiment of the present invention.

FIG. 3A Show wavelengths locking features using dual etalons.

FIG. 3B Show wavelengths locking features using dual etalons.

FIG. 3C Show wavelengths locking features using dual etalons.

FIG. 3D Show wavelengths locking features using dual etalons.

FIG. 3E Shows expected transmission curve for the FIG. 3 embodiment.

FIG. 4 Shows actual transmission curve at 1529.3 nm for the rubidiumcell laboratory demonstration.

FIG. 5 Shows wavelength bands in wide use for fiber optic communication.

FIG. 6A Show some atomic transmission for cesium and rubidium.

FIG. 6B Show some atomic transmission for cesium and rubidium.

FIG. 7A Show features of a prior a Faraday filter.

FIG. 7B Show features of a prior art Faraday filter.

FIG. 7C Show features of a prior art Faraday filter.

FIG. 8 Shows background spectral data.

FIG. 8A Show prior art Faraday and Voigt results.

FIG. 8B Show prior at Faraday and Voigt results.

FIG. 8C Show prior art Faraday and Voigt results.

FIG. 9 Shows an application of the present invention.

FIG. 10A Show etalon data.

FIG. 10B Show etalon data.

FIG. 10C Show etalon data.

FIG. 10D Show a laboratory demonstration of a signal laser lockingtechnique.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Prototype Results

Cesium Vapor Cell

FIG. 1 shows a prototype layout used by Applicants to demonstrate thepresent invention.

For this demonstration, a 10 mm long cesium cell, 4 containing cesiumvapor at 157° C. is placed in a magnetic field of 150 Gauss produced bytwo ring magnets, 6 (Dexter Permag PP96-1213-1). The cesium vapor wasexcited with a 852 nm pump beam to the Cs 6²P_(3/2) level as shown inFIG. 3 by pump laser 8 which is a diode laser supplied by SDL (now partof JDS Uniphase) (SDL 5722-H1). For this demonstration a tunableexternal cavity diode laser 10 (Near Focus Velocity 6326) is used forthe signal beam so that a wide spectral range could be examined. Thelaser is tunable within the range of 1446-1557 nm. The output beam islinearly polarized. A long pass filter 12 blocks virtually allwavelengths below about 1.0 μm including light at the pump beamwavelength of 852 nm and Wallaston polarizing prism 14 is arranged withits polarization direction perpendicular to the polarization directionof the signal beam from tunable laser 10. Neutral density filter 16reduces the intensity of light across a broad spectrum to match theintensity of the signal light operating range of the test instruments.Thus, substantially all light is blocked except light from laser 10which is polarization rotated within cesium cell 4.

FIG. 2 shows the results obtained by scanning the laser through therange of about 0.05 nm centered at 1469 nm. As is evident from thefigure almost 100% of the light with a wavelength band of less than 0.01nm is transmitted with substantially zero transmission outside a band ofabout 0.05 nm. The two side lobes shown in FIG. 2 would produce somenoise but in many application this would be insignificant.

Rubidium Vapor

A second bench top experiment was conducted using a rubidium cell. Thisexperiment was very similar to the cesium cell experiment and is shownin FIG. 1B. The only difference is that the vapor cell 4 is a rubidiumvapor cell and the pump laser is a 780 nm pump laser model Vortex 6013supplied by New Focus with offices in San Jose, Calif. and the half waveplate 9 is chosen to correspond to the 780 nm wavelength. The resultswere equivalent to those shown for the co-experiment. This experimentshows that an atomic line filter can be provided in accordance with thepresent invention to provide an excellent filter for the very importantC band of fiber optics communication. See FIG. 5.

Wavelength Locking

To fully utilize the excited atomic line filters of the presentinvention, the signal laser frequency must be stabilized within thetransmission bandwidth of the filters during operation which mayencounter temperature drift, vibration and mechanical shock. Effortshave been put forwards to stabilize the laser frequency, the success,however, is limited up to date. Earlier wavelength locker designdescribed in prior art patents cited in the Background section utilizedan extended cavity laser with an ALF inside the cavity. Applicants havediscovered that this design is sensitive to vibration and mechanicalshock. The laser is scanned in frequency to locate the peak ALFtransmission position. The laser is then periodically scanned and tunedto keep the frequency at the peak transmission position. Additionalcomplications results from the fact that the laser frequency is verysensitive to the laser diode temperature which is affected not only bythe environmental temperature, but also by the tuning of the lasercurrent.

Preferred embodiments of the present invention employ a pair ofmicro-etalons in order to provide frequency locking of the signal laser.Preferably the transmission peaks of the etalons differ by the fullwidth at the half maximum (FWHM) of the transmission band of the atomicline filer and the peak frequency of one of the etalon is located at oneside of the filter transmission band and the peak frequency of the otheretalon is located at the other side of the filter transmission band. Thelaser wavelength is locked at the middle of the pair of the transmissionbands. FIG. 10A shows the simulated etalon transmission spectra of apair of etalons by CVI Technical Optics. The peak transmission of eachof the etalons can be adjusted by changing the etalons spacing and/orcharging the angular position of the etalon relative to the sample beamas shown in FIGS. 10B and 10C. Etalon performance is also affected bytemperature and pressure changes.

Several Parameters need to be Considered in Designing an Etalon:

a) Transmission Wavelength and Free Spectra Range

An etalon is compose of two highly reflective parallel surfaces with themedium between the two surfaces either air or solid. Etalontransmissions are results of multiple beam interference. The transmittedwavelength is determined byλ=(2.n′.h.cos θ′)/mwhere, n′ is the index of refraction of the medium in the etalon gap; his the thickness of the etalon gap; θ′ is the refraction angle insidethe medium; and m is the interference order. The transmission peaks areequally spaced in frequency with frequency spacing, the so-called freespectra range (FSR), determined by C/(2.n′.h.cos θ′), where C is thespeed of light. The transmission wavelength for a given interferenceorder can be tuned by varying the angle of refraction, or the spacing ofthe etalon, or index of refraction of the material in the space betweenthe surfaces that material is typically a gas such as air and n isaffected by the pressure and temperature of the gas. FIGS. 10B and 10Cplot the frequency shift of the transmission band with angle ofrefraction and etalon spacing, respectively, for an air-spaced etalonwith 7.5 mm spacing or 20 GHz FSR at 852 nm central transmissionwavelength. To achieve sub 100 MHz resolution angle-tuning of thetransmission band, the required angular control accuracy is on the orderof hundreds of micro radian.b) FWHM and Finesse

The finesse (f) of the etalon defined as the FSR divided by the FWHM ofthe transmission bands is only determined by the reflectance (R) of theetalon $f = \frac{\pi\sqrt{R}}{1 - R}$

The larger the reflectance, the larger the finesse and thereforenarrower transmission bandwidth for a given spacing or FSR. To produce aFWHM of 0.5 GHz at 780 nm with a FSR of 25 GHz, the required reflectanceis about 94%. In practice, defects in Optics will deteriorate thefinesse of the etalons producing a somewhat wider transmission band.

c) Thermal Stability

Etalons can be air-spaced or solid. For better thermal performance,air-spaced etalons are the choice because the spacers can be made oftemperature insensitive materials such as Zerodur. The cavity may alsobe sealed to reduce index of refraction fluctuation with temperature.The quoted thermal stability form WavePrecision Inc. is −0.007 GHz/° C.for sealed air-spaced etalons made of Zerodur.

Preferred etalons are air-spaced with 7.690 mm spacing which results ina free spectra range (FSR) of 19.5 GHz. The reflectivity of the coatingis 90% at 780 nm, which leads to a finesse of 31. The corresponding FWHMof the transmission bands is about 0.7 GHz.

A schematic diagram of a wavelength locker for locking a diode laser isshown in FIG. 10D. Mini etalons and micro optics are used. The outputfrom laser diode 70 is first collimated. A few percent of the collimatedbeam will be then picked off by beam splitter 72 for wavelength locking.The sampled beam will be split into two beams, one of which will be sentto a small absorption cell 74 of atomic vapor for absolute wavelengthtuning of the laser wavelength, another beam will be sent to the etalonpair assembly 76A and 76B for wavelength locking signals from detectionD1, D2 and D3 are used for tuning the etalon on both sides of theabsorption peak. The whole package will be temperature-controlled with aTE cooler. The reader should not that once etalons have been tuned andlocked in place, the absorption cell may no longer be needed. Also, forlocking the beacon laser, the atomic line filter is preferably used forabsolute tuning.

Applications Laser Tracking

A preferred application of the present invention is for laser trackingof a moving target. In this case the moving target may utilize arelatively wide divergence beacon laser which is tracked by a trackingdetector which includes one or more filters such as one of thosedescribed above. The beacon laser system should be stabilize the beaconlaser at the specific wavelength band of the very narrow band filter. Inthis case, a very narrow band filter is needed so that reflected andscattered sunlight can be filtered out. FIG. 8 shows examples ofspectral background looking directly at the sun 80, looking at a sunlitcloud 82, scattering from a clear sky and emission from a clear sky. Atracking system must compete with this light and must produce asufficient signal to noise ratio to stay on track. FIG. 9 is a drawingshowing ground-to-air tracking. The technique used is to provide abeacon laser operating at or within a precise wavelength range with awide enough angle to be relatively certain of including the trackingsystem.

This provides the tracking system with a narrowband filter which will beas efficient as possible at the beacon laser wavelength and as narrowenough so as to discriminate sufficiently against background light sothat the signal to noise ratio is large enough to permit tracking.

Dielectric optical bandpass filters typically achieve bandwidths of 0.5%to 1% of the center wavelength, and suffer from increased transmissionloss with decreasing bandwidth, limited acceptance angle, and wavelengthshift with temperature. Atomic line filters have demonstrated superiorperformance with bandwidths less than 0.02 nm, greater than 30 degreeacceptance angle, and little or no dependence upon temperature. However,these filters are limited to wavelengths in the visible and near visible(<853 nm). A new excited state Faraday resonance filter of the presentinvention extends conventional proven atomic line filter techniques tothe 1550 nm region. This filter provides increased background reductionof >100 over the best optical filters without the angular field of viewand temperature shift limitations of dielectric optical filters.

For negligible electronic noise, an FPA used in beacon acquisition willhave an SNR given by${SNR} = \lbrack \frac{\eta\quad P_{s}}{2{hv}\quad\Delta\quad{f( {{1 + P_{b}},P_{s}} )}} \rbrack^{1/2}$where P_(s) is the per pixel detected signal power, P_(b) is the perpixel background power and Δf is the sensor integration bandwidth. Forbackground limited performance it can be shown that the IFOV$\theta = ( \frac{P_{s}/A}{{\Delta\lambda}\quad L_{\lambda}} )^{1/2}$when P_(s)=P_(b), Δλ is the bandpass,

A is the pixel area, and L_(λ) is the background radiance (W/cm²/SR/μ).This important result shows that decreasing the optical filter bandwidthby a factor of 100 will allow the received beacon power density to bedecreased by 100 for the same sensor field of view and background powerdensity. Or, alternatively, will allow 100× more solid angle to beilluminated thereby decreasing search time.

Dielectric bandpass filters having narrow bandwidths will have centertransmission wavelength shift with angle of incidence (θ) proportionalto sin²θ. For a SOTA filter of 0.1% bandwidth, this shift would exceedthe bandwidth for an angle of incidence of 10 degrees (FIG. 2). Thus thenew ultra-narrow filters being made for fiber DWDM systems (ie 0.1% BW)will not meet the viewing angle requirements needed for an acquisitionsensor. The 1.55 micron atomic line filter described above can readilyachieve 1/100 of this bandwidth (0.02 nm @ 1550 nm) with greater than 30degree acceptance angle and 80% transmission.

Preferred Embodiment

Details of the principal components and features of a preferredembodiment of the present invention are shown in FIGS. 3, 3A, 3B, and3C. This embodiment includes a beacon laser unit 30 that might belocated on a moving platform such as an unmanned aerial vehicle such asthose shown in FIG. 9. It also includes excited state atomic line filterunit 32 comprising rubidium ALF 34 and pump laser unit 36.

The beacon laser unit comprises diode signal laser 38, frequency lockingetalons 40A and 40B, and detectors 42A and 42B providing frequencylocking signals to processor 44 which utilizes those signals to controlthe current to signal laser 38 through current control 46. Processor 44also maintains control of the diode temperature and the chambertemperature through thermoelectric coolers (not shown) and diodetemperature control unit 46 and chamber temperature control unit 48.

Pump laser unit 36 provides a laser beam locked precisely to awavelength λ=780.027 nm to excite rubidium vapor in vapor cell 70 to the5²P_(3/2) excited state as shown in FIG. 6B. Controls for this laser aresimilar to those for beacon laser 30. The unit includes diode pump laser50, frequency locking etalons 52A and 52B and detectors 54A and 54Bproviding frequency locking signals to processor 56 which utilize thesesignals to control the current to pump laser 50 through current control58. Processor 56 also maintains control of the diode temperature and thechamber temperature through thermoelectric coolers (not shown) and diodetemperature control unit 57 and chamber temperature control unit 59. Theoutput beam 60 of pump laser unit 36 is co-aligned with beacon beam 62from beacon laser 30 using dichroic mirror 64.

Rubidium atomic line filter 34 comprises rubidium cell 70, two ringmagnets 72 producing a 150 Gauss co-axial magnetic field through cell70, cross polarizers 74 and 76, long pass filter 78 and detector 80.Temperature controller 82 controls the temperature of cell 70 viarubidium heater 84. Cold finger 86 provides a condensation locationwithin the cell to maintain a rubidium vapor-liquid equilibrium withincell 70. The signal from detector as shown at 88 may be utilized bypointing equipment (not shown) to maintain a telescope such as thoseshown in FIG. 9 pointed at beacon laser 30.

FIG. 3A provides a qualitative representation of the output of detectionD1 and D2 of beacon signal laser 30 as a function of the outputwavelength of beacon signal laser 38. FIG. 3B shows qualitatively therelationship between the ratios of the detector signals to thewavelength within the narrow spectral range of about 1529.29 nm to about1529.31 nm. Current to diode laser 38 is adjusted to maintain'the outputof beacon laser 38 precisely within the transmission band of filter unit32 as shown in FIG. 3F.

FIGS. 3C and 3D are similar graphs demonstrating the control of thewavelength of the pump laser beam from pump laser unit 36. Thiswavelengths can also be controlled to a precession of about ±0.0005 nm.Specification for pump 36 laser provide a response of 0.0034 nm permilliamps permitting wavelength precision to about +0.0005 nm.

Alignment and Packaging Procedure

The alignment and packaging process for both the pump laser and thebeacon laser should be performed in a well controlled environment sothat the laser wavelength will be stable to about 100 MHz even withoutwavelength locking in a period of one hour or so. At first allcomponents other than the two etalons will be aligned and fixed topositions either mechanically or using thermal/UV curing epoxy. The twoetalons will be attached to two precession (100 μ radian or betterresolution) angular alignment stages, respectively, by mechanical orvacuum means. The beacon laser is scanned to obtain an ALF transmissionspectrum and the pump laser is scanned to obtain an absorption spectrum.For the beacon laser, the two etalons will be angularly-tuned so thattheir transmission spectra overlaps at the center of the ALF spectrum.For the pump laser, the transmission spectrum of the two etalon overlapat the center of the absorption spectrum. The etalons will be fixed topositions with UV epoxy.

Control Electronics and Operation Procedure

A microprocessor will be used to control the laser current and processthe outputs from the two etalon-photodiodes. The diode laser TE cooler,the atomic vapor cell heater, and the TE cooler for the whole packagemay also be controlled with the microprocessor.

To operate the wavelength locker, at first all temperature controlsshould be stabilized with the laser current ramped up to the operationvalue. The laser wavelength will then be locked to the position bybalancing the light output from the two etalons through the tuning ofthe laser current with the microprocessor.

Packaging

The etalons will be made of temperature-insensitive materials and thewhole package may be temperature-controlled as well. So thermalstability should not be a critical issue. However, as shown in FIG. 10B,the peak transmission frequency of the etalons is very sensitive toangle of incident. To lock the laser frequency better than a fewhundreds of MHz, the incident beam should be stabilized better than 1mradian. To reduce spatial jitter of the incident beam from the beaconlaser to the wavelength locker, the whole package should be made ofsmall size components with dimensions on the order of a few centimeters.Furthermore, the beacon laser and the line locker should be integratedinto one small package.

While the above description describes specific preferred embodiment ofthe present invention, person skilled in this art should understand thatmany changes and modifications could be made within the scope of thepresent invention. For example, vapors other than rubidium and cesiumcould be used Specifically, other alkali metals are good choices, wherea desirable resonant frequency exists between two excited states andwhere the vapor can be pumped to the first excited state. Goodapplication of the present invention includes tracking a transceiver ina laser communication system. Also, the system may be used to track hotobjects such as bullets or other missiles emitting radiation at infraredwavelengths.

Another application is for laser radar systems where eye safety is aconcern. For these reasons the reader should determine the scope of theinvention from the appended claims and their legal equivalence.

1. An atomic line filter comprising (A) a metal vapor cell having anoptical entrance port and an optical exit port and containing a metalvapor defining a first excited energy state, defining a first resonantfrequency, and a second excited energy state and having at least oneabsorption line, at or near a desired filter wavelength, said absorptionline being equal, in energy, to a difference between the second excitedstate and the first excited state; (B) at least one magnet for imposingon said metal vapor a magnetic field to produce polarization rotationnear said at least one absorption line; (C) a first polarizer positionedto block polarized light at a first polarization from entering saidvapor cell through said entrance port; (D) a second polarizer orientedperpendicular to said first polarization and located across the path oflight exiting said exit port so as to block light exiting said vaporcell that is polarized perpendicular to said first polarization; (E) apump light source for producing light at said first resonant frequencyfor pumping said metal vapor from said ground state to said firstexcited state; wherein said metal vapor under the influence of saidmagnetic field produces a polarization rotation of light within a narrowspectral band near said absorption line permitting light within thisspectral band to pass through said second polarizer whereas all light ina much wider spectral range is not rotated in polarization and isblocked by either the first polarizer or the second polarizer.
 2. Thefilter as in claim 1 wherein said pump light source is a pump lasersystem.
 3. The filter as in claim 2 wherein said pump laser system is adiode laser system producing a pump beam
 4. The filter as in claim 3wherein said diode laser further comprises two frequency locking etalonsand two frequency detectors for monitoring sample portions of said pumpbeam.
 5. The filter as in claim 4 wherein said two etalons are tuned toproduce transmission peaks on both sides of said first resonantfrequency and feedback control to maintain said diode laser operation atsaid resonant frequency based on input signals from said two frequencylocking detectors.
 6. The filter as in claim 1 wherein said metal vaporis rubidium.
 7. The filter as in claim 1 wherein at least one magnet istwo permanent magnets.
 8. The filter as in claim 7 wherein said twopermanent magnets are two ring magnets.
 9. The filter as in claim 1wherein said at least one magnet is an electromagnet.
 10. The filter asin claim 1 wherein said vapor cell defines beam direction and at leastone magnet is oriented to produce a magnetic field parallel to said beamdirection.
 11. The filter as in claim 1 wherein said vapor cell definesa beam direction and said at least one magnetic is oriented to produce amagnetic field perpendicular to said beam direction.
 12. A trackingsystem comprising: (A) a metal vapor cell having an optical entranceport and an optical exit port and containing a metal vapor defining afirst excited energy state, defining a first resonant frequency, and asecond excited energy state and having at least one absorption line, ator near a desired filter wavelength, said absorption line being equal,in energy, to a difference between the second excited state and thefirst excited state; (B) a beacon laser system for producing a beaconlaser beam at a wavelength within said narrow spectral band.
 13. Thetracking system as in claim 12 wherein said beacon laser is configuredto produce a beacon laser beam with a divergence of at least 20 degrees.14. The tracking system as in claim 12 wherein said beacon laser usingsystem comprises two frequency locking etalons and two frequency lockingdetectors for monitoring sample portions of said beacon laser beam. 15.The filter as in claim 14 wherein said two etalons are tuned to producetransmission peaks on both sides of said first resonant frequency andfeedback control to maintain said diode laser operation at said resonantfrequency based on input signals from said two frequency lockingdetectors.